Author:John Harrod, Delaware Nature Society

Reference:Swearingen, J., K. Reshetiloff, B. Slattery,and S. Zwicker. 2002. Plant Invaders ofMid-Atlantic Natural Areas. National ParkService and U.S. Fish & Wildlife Service,82 pp.

Project participants:Sue Barton, University of DelawareValann Budischak, University of DelawareJen Gochenaur, Delaware Nature SocietyFaith Kuehn, Delaware Department ofAgricultureGary Schwetz, Delaware Center forHorticulture

Photographs provided by:Olin AllenDave ProJim WhiteSusan Barton

Graphic design by:Carrie Finnie

Funding provided by:National Urban and Community ForestryAdvisory Council (NUCFAC)

Supported by:Delaware Center for HorticultureDelaware Department of AgricultureDelaware Invasive Species CouncilDelaware Nature SocietyDelaware Nursery and Landscape Assoc.University of Delaware

Acknowledgements

controllingBackyard Invaders

2 3

What are Invasive Plants?

Invasive plants quickly overwhelm and displace existing native plants by reduc-ing the availability of light, water, nutrients and space. They have few, if any, natu-ral controls to keep them in check. Ecologists now rank invasion by exotic plants,animals, and pathogens second only to habitat loss as a major threat to local biodi-versity.

Invasive plants may be introduced by accident or intentionally to control erosion, provide wildlife food and habitat, or for ornamental value in gardens.Accidental introductions occur when people and goods travel worldwide. Packingmaterial can harbor seeds or plant parts. Japanese stilt grass, now a widely escapedgroundcover in woodland edges, is a prime example.

Invasive plants can be divided into two categories—(1) plants that were intro-duced either intentionally or accidentally but are no longer sold (i.e. multiflorarose, stilt grass) and (2) ornamental plants still grown and sold.

This brochure focuses primarily on invasive plants no longer sold. The goalis to guide home and property owners in the identification and control of aggres-sive invasive plants. Not only will control improve the diversity of native andnon-invasive plants, but it will also improve habitat and help prevent the spreadof invasive plants to neighboring areas.

Controlling Invasive Plants

Through the Plants for a Livable Delaware effort, a team of scientists, natural-ists, and nursery industry professionals identified mechanical and chemical controlmethods for some of the most aggressive and prevalent invasive plants in Delaware.These control methods are listed in this brochure under the individual invasiveplant species.

Whenever possible use mechanical control (i.e. digging, cutting) as your firstline of defense. Save chemicals as the last alternative, because their introductioncan have long term and/or unintended effects on the environment.

Mechanical control treatment is similar in many plant species. The basic strate-gy for trees, shrubs, and vines is to cut off the trunk or stem and then removeand/or kill the roots. Although many species will re-sprout from the cut stump orstem, continued cutting of the new growth will eventually “starve” the roots, inmost cases. Control herbaceous perennial plants by removing the entire plant,including roots, before flowering. Prevent annual plant seed production by remov-ing plants, or by mowing or clipping flower heads before full bloom and/or seedset. Keep in mind that mowing can also control or eliminate desirable native vege-tation, so consider a more selective control method for sites where desirable plantsare already dominant. Invasive plants are opportunistic and take over a site byspreading much faster than local native plants, especially when soil, drainage, andlight patterns are disturbed. Avoid creating excessive disturbance to plants in theimmediate area.

Cover photo background: masses of Autumn olivedominate the vegetation in this Delaware roadside.

controllingBackyard Invaders

4 5

If chemical controls are the best solution for your situation, choose an herbicide that is non-persistent in the environment. Follow the manufacturer’s instructions. The recommended chemicals for invasive plant control may require assistance from a professional with a pesticide applicator's license, as some of the chemicals are restricted.Contact the of the Delaware Department of Agriculture pesticide section at 302-698-4570 for a list of certified professional applicators.

Sometimes, neither mechanical nor chemical control alone is sufficient, and a com-bination of controls is necessary. Mechanical control may remove only the top of theplant leaving the roots in place or chemical control may kill the plant but leave unsight-ly dead stems. An example of a combined control method is cutting a woody plantdown to the stump and treating the stump with a systemic herbicide. Systemic herbi-cides move through a plant’s vascular system killing the entire plant.

Before you begin removing invasive plants, it is important to have a plan in place torestore the disturbed habitat. Hedgerows and tangles of vegetation containing invasiveplants often are sites of nesting birds and may be havens for small mammals. The besttime to remove invasive plants from hedgerows is from September through March whenbirds are not nesting.

For most effective control of invasive plants covered in this brochure use a 1% solution ofRoundup Pro®. In the case of Japanese knotweed, use a 2% solution of Roundup Pro®.

What do I do once I remove invasive plants?

Avoid bare ground. Promptly replant disturbed areas as soon as possible. Usenative plants in natural areas and stream corridors, or non-invasive ornamentals ina yard or garden. Acquire native plant lists from the Delaware Nature Society,Delaware Native Plant Society, or the Plants for a Livable Delaware companionbooklet, which lists alternatives to invasive plant species. You can also monitor nat-ural succession of native plants to reclaim a vacant space.

Prevent potential erosion problems caused by plant removal by covering anyunplanted ground with salt hay, not straw. Salt hay, unlike straw, does not containweed seeds. Mulch flower beds to cover exposed bare soil and reduce invasive weedseedlings. Maintain the area by controlling any invasive plants that come up,reducing the competition with desirable plants.

Plants for a Livable Delaware is a campaign to identify and promote superiorplants that thrive without becoming invasive.

A Livable Delaware plant must• Possess adaptable characteristics to landscape situations (i.e. drought resistant,

tolerant of poor soils, etc.)• Pose no potential threat as an invasive plant• Have no serious disease or insect problems• Be hardy to DelawareLivable Delaware Plants are identified at some garden centers, public gardens

and natural areas in Delaware. They provide safe and desirable alternative to inva-sive plants.

7

Trees & Shrubs

ProblemMultiflora rose(Rosa multiflora)

Originally promoted for erosion controland wildlife cover, multiflora rose is a shrubthat spreads by seeds, roots, and layering(the process of rooting branches, twigs orstems still attached to the parent plant). Itsmothers native vegetation with its densethickets of arching or trailing thorny stemsthat grow up to 13 feet tall. It is found infields, forest, meadows, and riparian areas,and is a dominant component of roadsidevegetation.

Identification: Multiflora rose producesnumerous fragrant clusters of white flowersin June that form small, red hips (fruits).Twigs are reddish in winter.

Control: Remove plants by digging outthe crown (where roots join the stem). Cutstems can be treated with systemic herbi-cide to kill roots. A glyphosate-based herbi-cide, like Roundup Pro®, can be applied tofoliage in April and May before most nativeplants have produced leaves.

6

ProblemAutumn olive(Elaeagnus umbellata)

Autumn olive was originally used forwildlife habitat, erosion control, and as anornamental. It is a large shrub reaching 20feet and is spread by seeds. It toleratesdrought and poor soil, which allows it to grow in meadows and open woodlands.It creates dense shade and excludes nativetrees and shrubs.

Identification: The underside ofautumn olive twigs and leaves are coveredwith small silvery scales. It has fragrantsmall white flowers in spring and red fruitsripening in September and October.

Control: Remove small plants andseedlings by hand. For mature plants, cuttrunks and treat fresh cut trunks with a systemic herbicide. Treat new sprouts withherbicide as needed.

Trees & Shrubs

Autumn olive with fruit

Silvery scales of autumn olive

Autumn olive flowering Multiflora rose invading open space

Multiflora rose stem

Multiflora rose hips

Multiflora rose flowers

98

ProblemPorcelain berry(Ampelopsis brevipedunculata)

Porcelain berry was brought to theUnited States as a landscape plant. It is awoody climbing vine that spreads by seedand through layering. Porcelain berry growswell in dry or moist areas, smotheringshrubs and small trees in upland areas andaround stream banks and pond margins.

Identification: The leaves and growthhabit are similar to grapevine. Porcelainberry has small inconspicuous flowers thatturn into smooth fleshy berries in shades ofblue, lavender, pink, turquoise, and yellow.

Control: Remove smaller vines androots by hand. For older plants, cut stemsand treat cut trunks with a systemic herbi-cide.

Vines Vines

ProblemOriental bittersweet(Celastrus orbiculatus)

Oriental bittersweet was introduced asan ornamental plant and is still sold insome places today. It is a woody climbingvine that spreads by seeds. It grows infields, forest edges, and gaps in woodlandcanopys where it smothers all layers of vegetation from tall trees to small shrubs.

Identification: Small green flowers inleaf axils turn to orange fruits that splitopen in autumn to reveal fleshy red seeds.It has round smooth leaves and scaly bark.

Control: Remove smaller vines androots by hand. On older plants, cut trunksand treat with a systemic herbicide to killthe roots.

Porcelain berry fruit

Porcelian berry leaves

Oriental bittersweet with unripe fruit

Oriental bittersweet trunk

Oriental bittersweet fruit

1110

ProblemJapanese honeysuckle(Lonicera japonica)

Japanese honeysuckle was introducedmore than 100 years ago for erosion controland wildlife food and cover. It is a woodytrailing or climbing vine that spreads vege-tatively and through seeds. It grows in sunto medium shade in disturbed areas includ-ing fields and forests, and smothers shrubsand small trees.

Identification: Japanese honeysuckle has white fragrant flowers that age to a yellowish color and semi-evergreen ovalleaves. Round black berries develop in late summer.

Control: Remove smaller vines androots by hand. Mow twice a year, in Julyand September, to limit vegetative spread.Spray plants with a glyphosate-based herbi-cide, like Roundup Pro®, in mid-November to early winter when honeysuck-le still has green leaves and other plants aredormant.

Vines Perennials

ProblemGarlic Mustard(Alliaria petiolata)

Garlic mustard is a biennial (lives twoyears, normally producing flowers and seedsthe second year) herb that was introducedfor food and medicinal purposes. Spreadingby seed, it grows in shaded woods or onroadsides, displacing native wildflowers.

Identification: Toothed, heart-shapedleaves smell like garlic when crushed.During the first year small rosettes growclose to the ground. In the second year itsends up 1-3’ flower stalks with delicate 4-petaled white flowers.

Control: Remove plants by hand dur-ing flowering in June and early July to pre-vent seed production. Destroy flower stalksto prevent seed from forming on plants castaside. Large patches can be mowed close tothe ground while in full bloom to preventseed development. Spray year one rosetteswith a glyphosate-based herbicide, likeRoundup Pro®, in mid-November to earlywinter when other plants are dormant andgarlic mustard is still green.

Japanese honeysuckle fruit

Garlic mustard rosettes

Garlic mustard flowering

Garlic mustard in natural area

Japanese honeysuckle flowering

1312

ProblemCanada Thistle(Cirsium arvense)

Canada thistle is an herbaceous perenni-al that is considered one of the most tena-cious agricultural weeds by Canada and theU.S. It grows in full sun in dry, disturbedareas and agricultural fields. Spreading byseeds and roots, it crowds out native mead-ow vegetation.

Identification: Canada thistle hasnumerous small purple flower heads on 1’to 5’ plants with prickly leaves. The seedsare dandelion-like, white feathery tufts.

Control: Cut or mow down plants inlate June when the plant is beginning toflower to prevent seed production. Targetedapplication of a glyphosate-based herbicide,like Roundup Pro®, or a broadleaf specificherbicide, like Stinger™ or Confront®, iseffective. Multiple applications may be nec-essary.

Perennials Perennials

ProblemJapanese Knotweed(Polygonum cuspidatum)

Japanese knotweed was introduced asan ornamental plant and has been used forerosion control. Knotweed regenerates easi-ly from small vegetative pieces and seeds,quickly forming dense thickets. It is anherbaceous perennial that can grow to over10’ tall and is commonly found in sun ormedium shade near water sources andwaste areas.

Identification: Japanese knotweed has ahollow stem and large, oval leaves withpointed tips and square bases. Long, lacyclusters of tiny white flowers appear at theend of summer.

Control: Pulling young plants is effec-tive with repeat visits to remove re-sproutsfrom stem fragments and root pieces.Cutting back in June followed by an appli-cation of a glyphosate-based herbicide, likeRoundup Pro®, during September andearly October is effective. Cutting and cov-ering with black weed plastic may kill smallinfestations. Control should be repeatedover several years to be fully successful.

Canada thistle with seeds

Canada thistle flowering New growth of Canada thistle

Japanese knotweed flowering

Japanese knotweed

Japanese knotweed mass

1514

ProblemLesser Celandine (Ranunculus ficaria)

Lesser celandine, which was introducedas an ornamental plant and is still commer-cially available, is an herbaceous perennialalso known as fig buttercup. It is predomi-nantly found in moist, forested floodplains.Lesser celandine emerges in early springbefore most native spring ephemerals(plants, such as jack-in-the-pulpit, thatemerge, bloom and die back during oneseason). Spreading primarily through bul-blets and tubers, it crowds out ephemeralsby creating a dense carpet across theground.

Identification: Lesser celandine grows to5 inches and has shiny kidney shapedleaves. It looks similar to the native marshmarigold (Caltha palustris) which grows to24 inches and does not produce tubers orform a continuous carpet of growth.

Control: Remove the entire plant and asmany tubers as possible by hand digging forsmall infestations. For large areas, systemicherbicides formulated for wet areas, likeRodeo™, should be applied in late winter-early spring when it emerges and beforeother vegetation is showing green. Add asurfactant to help the herbicide adhere tothe leaves.

Perennials Perennials

ProblemStilt Grass (Microstegium vimineum)

Stilt grass is an annual that probablyescaped from its use as a packing material.It grows in full sun to deep shade, crowd-ing out native wildflowers and grasses. Stiltgrass prefers moist conditions and spreadsvegetatively and by seed, especially in dis-turbed areas.

Identification: Narrow, elongated leavesare alternately spaced along the fine,sprawling stems that resemble miniaturebamboo. Stilt grass can be differentiatedfrom other grasses by the presence of a sil-ver stripe on the midrib of the grass blade.

Control: Remove plants by hand. Forlarge areas, mow in August /Septemberwhile the plant is in flower to prevent seedproduction. Monocot (i.e. grasses, sedges,yuccas) specific herbicides may be used inAugust for extensive infestations whenmechanical methods are impractical andwhen the nearby desirable species arebroadleaved plants. Pre-emergent herbi-cides, applied in early spring, will preventstilt grass seeds from germinating.

Lesser celandine flowering

Lesser celandine on forest floor

Stilt grass shoots

Stilt grass seed

Stilt grass leaves

17

Perennials

ProblemJapanese Hops(Humulus japonicus)

Japanese hops is an herbaceous annualvine that is still currently sold for ornamen-tal and herbal purposes. It is primarily aweed of abandoned fields and open dis-turbed areas, and rapidly climbs or trailsalong the ground displacing native vegeta-tion.

Identification: Japanese hops is coveredwith small velcro-like barbs that are turneddownward on the stem. Its leaves can causedermatitis in sensitive individuals. It lookssimilar to wild cucumber, which has ten-drils and does not have barbs.

Control: Pull plants anytime beforeSeptember to prevent seed production.When pulling the plants, attempt toremove as much of the rootstock as possibleto reduce resprouting. Spot applications ofa glyphosate-based herbicide, like RoundupPro®, prior to flowering, should damagethe plant enough so it will not be able toflower and set seed. Japanese hops seed istypically exhausted in three years. The firsttwo years of control will be the most time-consuming. After that, the number ofplants will drop dramatically.

16

ProblemMile-A-Minute(Polygonum perfoliatum)

Mile-a-minute is an herbaceous annualvine that blankets and smothers native vege-tation with its rapidly growing trailingstems. It spreads by seed and grows in dis-turbed, moist areas in sun or part shade. Ithas been observed overtopping kudzu andJapanese honeysuckle.

Identification: Mile-a-minute has trian-gular leaves and funnel shaped sheaths thatwrap around the stem. It possesses sharpdownward pointing barbs, lending to itsother common name devil’s tear-thumb. Itproduces a spike of small green flowers fol-lowed by round blue berries.

Control: This plant is easily removed byhand in July and August before the seedsets. Use pre-emergent herbicides in theearly spring to reduce mile-a-minute seedgermination in areas of known infestation.

Perennials

Mile-a-minute fruit

Mile-a-minute with one season’s growth

Mile-a-minute triangular leaf and barbs

Japanese hops

Japanese hops leaves

Japanese hops vine

controllingBackyard Invaders

18 19

Widespread and Invasive• Norway maple

Acer platanoides• Common reed

Phragmites australis• Hydrilla

Hydrilla verticillata• Morrow’s honeysuckle

Lonicera morrowii• Sweetautumn clematis

Clematis terniflora• Privet

Ligustrum species• European sweetflag

Acorus calamus• Wineberry

Rubus phoenicolasius

Restricted and Invasive• Japanese barberry

Berberis thunbergii• Periwinkle

Vinca minor• Burning bush

Euonymus alata• Bradford pear

Pyrus calleryana ‘Bradford’• Marsh dewflower

Murdannia keisak• Purple loosestrife

Lythrum salicaria• Reed canarygrass

Phalaris arundinacea• Amur honeysuckle

Lonicera maackii• Tartarian honeysuckle

Lonicera tatarica• Tree of heaven

Ailanthus altissima• Spotted knapweed

Centaurea biebersteinii

Restricted and Potentially-Invasive• Butterfly bush

Buddleia davidii

Other Threatening Alien Species

In addition to the highly invasive plants pictured in this brochure, the follow-ing list of plants, provided by the Delaware Invasive Species Council, are knowninvasive species. Restricted and invasive plants are equally problematic; however,they have a more localized distribution in Delaware. Some are still sold in manynursery and garden centers. Alternatives to some of these invasive plants still grownand sold can be found in the Plants for a Livable Delaware companion booklet.

The University of Delaware is committed to assuring equal opportunity to all persons and does not discriminate onthe basis of race, color, gender, religion, ancestry, national origin, sexual orientation, veteran status, age, or disabilityin its education programs, activities, admissions, or employment practices as required by Title IX of the EducationAmendments of 1972, Title VI of the Civil Rights Act of 1964, the Rehabilitation Act of 1973, the American withDisabilities Act, and other applicable statutes. Inquiries concerning these statues and information regarding campusaccessibility should be referred to the Affirmative Action Office, 305 Hullihen Hall, (302) 831-2835 (voice), (302)831-4563 (TDD).

Plants for a Livable Delaware companion brochurelists great alternatives to some widely planted invasive species. Pick up a copy atyour Delaware Cooperative Extension office (New Castle County, 302-831-2506; Kent County, 302-730-4000; Sussex County, 302-856-7303); The DelawareDepartment of Agriculture, 1-800-282-8685; The Delaware Center forHorticulture, 302-658-6262; or the Delaware Nature Society, 302-239-2334.